1. Field of the Invention
The present invention relates to a dielectric ceramic composition having reduction resistance and a multilayer ceramic capacitor or electronic device using such a dielectric ceramic composition.
2. Description of the Related Art
An electronic device constituted by a multilayer ceramic capacitor is required to be high in relative dielectric constant, long in insulation resistance IR life, and good in DC bias characteristic (little change of relative dielectric constant along with time) and also good in temperature characteristics. In particular, depending on the application, it is required to have flat temperature characteristics under stringent conditions. In recent years, multilayer ceramic capacitors have come to be used in engine electronic control units (ECU), crank angle sensors, antilock braking system (ABS) modules, or other various types of electronic apparatuses mounted in automotive engine compartments. These electronic apparatuses are for ensuring stable engine control, drive control, and brake control, so the temperature stability of the circuits has to be good.
The environment in which these electronic systems are used falls in temperature to −20° C. or so or less in the winter in temperate regions. Further, after the startup of the engine, in the summer, the temperature is expected to rise to +130° C. or more. Recently, the wire harnesses connecting electronic apparatuses and their controlled equipment have been reduced as a general trend. The electronic apparatuses are sometimes installed outside the vehicles as well. Therefore, the environment in which the electronic apparatuses are placed has become increasingly harsh. Therefore, the capacitors used in these electronic apparatuses have to have flat temperature characteristics over a broad temperature range. Specifically, it is not enough that the capacity-temperature characteristic satisfy the X7R characteristic of the EIA standard (−55 to 125° C., ΔC/C=±15% or less). The X8R characteristic (−55 to 150° C., ΔC/C=±15% or less) of the EIA standard must be satisfied by the dielectric ceramic composition.
Several dielectric ceramic compositions have been proposed for satisfying the X8R characteristic.
Japanese Patent Publication (A) No. 10-25157 and Japanese Patent Publication (A) No. 9-40465 propose dielectric ceramic compositions having BaTiO3 as their main ingredients wherein the X8R characteristic is satisfied by having the Ba in the BaTiO3 substituted by Bi, Pb, etc. so as to shift the Curie temperature to the high temperature side. Further, selecting a BaTiO3+CaZrO3+ZnO+Nb2O5 system formulation to satisfy the X8R characteristic has also been proposed (Japanese Patent Publication (A) No. 4-295048, Japanese Patent Publication (A) No. 4-292458, Japanese Patent Publication (A) No. 4-292459, Japanese Patent Publication (A) No. 5-109319, and Japanese Patent Publication (A) No. 6-243721).
However, each of these formulations uses the highly volatile Pb, Bi, or Zn, so requires firing in the air or another oxidizing atmosphere. For this reason, there is the problem that the internal electrodes of the capacitors cannot be made using inexpensive Ni or other base metals and require use of Pd, Au, Ag, or other expensive precious metals.
As opposed to this, for the purpose of obtaining a high dielectric constant, satisfying the X8R characteristic, and enabling firing in a reducing atmosphere, the assignee has already proposed the following dielectric ceramic composition (Japanese Patent Publication (A) No. 2000-154057). The dielectric ceramic composition described in Japanese Patent Publication (A) No. 2000-154057 has at least a main ingredient constituted by BaTiO3, a first sub ingredient including at least one oxide selected from MgO, CaO, BaO, SrO, and Cr2O3, a second sub ingredient expressed by (Ba,Ca)xSiO2+x (where, x=0.8 to 1.2), a third sub ingredient including at least one oxide selected from V2O5, MoO3, and WO3, and a fourth sub ingredient including an oxide of R1 (where, R1 is at least one type of element selected from Sc, Er, Tm, Yb, and Lu), wherein the ratios of the sub ingredient with respect to 100 moles of the main ingredient are first sub ingredient: 0.1 to 3 moles, second sub ingredient: 2 to 10 moles, third sub ingredient: 0.01 to 0.5 mole, and fourth sub ingredient: 0.5 to 7 moles (where the number of moles of the fourth sub ingredient is the ratio of R1 alone).
Further, the assignee has already proposed the following dielectric ceramic composition (Japanese Patent Publication (A) No. 2001-192264). The dielectric ceramic composition described in this Japanese Patent Publication (A) No. 2001-192264 has a main ingredient including barium titanate, a first sub ingredient including at least one oxide selected from MgO, CaO, BaO, SrO, and Cr2O3, a second sub ingredient including silicon oxide as a main ingredient, a third sub ingredient including at least one oxide selected from V2O5, MoO3, and WO3, a fourth sub ingredient including an oxide of R1 (where, R1 is at least one element selected from Sc, Er, Tm, Yb, and Lu), and a fifth sub ingredient including CaZrO3 or CaO+ZrO2, wherein the ratios of the ingredients with respect to 100 moles of the main ingredient are first sub ingredient: 0.1 to 3 moles, second sub ingredient: 2 to 10 moles, third sub ingredient: 0.01 to 0.5 mole, fourth sub ingredient: 0.5 to 7 moles (where the number of moles of the fourth sub ingredient is the ratio of R1 alone), and fifth sub ingredient: 0<fifth sub ingredient≦5 moles.
In each application of the assignee explained above, the ratio of the MgO or other first sub ingredient with respect to 100 moles of the main ingredient was 0.1 mole or more.
Further, the assignee has already proposed the following dielectric ceramic composition (Japanese Patent Publication (A) No. 2002-255639). The dielectric ceramic composition described in this Japanese Patent Publication (A) No. 2002-255639 has at least a main ingredient including barium titanate, a first sub ingredient including an oxide of AE (wherein AE is at least one element selected from Mg, Ca, Ba, and Sr), and a second sub ingredient including an oxide of R (wherein R is at least one element selected from Y, Dy, Ho, and Er), wherein the ratios of the sub ingredients with respect to 100 moles of the main ingredient are first sub ingredient: 0 mole<first sub ingredient<0.1 mole and second sub ingredient: 1 mole<second sub ingredient<7 moles.
Note that a dielectric ceramic composition not having barium titanate as its main ingredient, but having barium calcium titanate as its main ingredient, yet having a high dielectric constant, satisfying the X8R characteristic, and enabling firing in a reducing atmosphere has also been proposed (Japanese Patent Publication (A) No. 11-302072).
However, in each of the latter four publications, while the dielectric constant is high, the X8R characteristic is satisfied, and firing in a reducing atmosphere is possible, the IR temperature dependency from room temperature to a high temperature is poor and actual use as a product has been difficult.